Reversible hydrogen storage composition

ABSTRACT

Hydrogen storage compositions which liberate hydrogen readily and which are readily regenerated from a dehydrogenated state formed by liberation of hydrogen are derived from an AlH 3 -based complex hydride incorporating a member selected from a metalloid such as B, C, Si, P and S, a metal such as Cr, Mn, Fe, Co, Ni, Cu, Mo, Zn, Ga, In and Sn, a metal which forms a stable hydride such as Be, Mg, Ca, Ti, V, Y, Zr and La and a second AlH 3 -based complex hydride.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This Application is a Continuation of PCT/CA 98/00947, filed Oct.7, 1998, in which the United States of America was designated andelected, and which remains pending in the International phase until Apr.7, 2001.

TECHNICAL FIELD

[0002] This invention relates to a hydrogen storage composition and to amethod of supplying hydrogen.

BACKGROUND ART

[0003] Amongst the known metal hydrides, only a few exhibit propertiessuitable for reversible hydrogen storage. Reversibility of hydrogensorption by a metal hydride means the capability to absorb and desorbhydrogen under practical conditions of hydrogen pressure andtemperature. Most hydrides are either too stable for hydrogenationcycling so that absorption is easy but desorption of hydrogen requiresvery high temperatures; or too unstable so that desorption occursreadily, but absorption requires extremely high hydrogen pressure.

[0004] Stable metal hydrides include such compounds as LiH, which meltsat the temperature of 680° C. but decomposes at an even highertemperature of 720° C., TiH₂, CaH₂ and ZrH₂, which have to be heated upto 650° C., 600° C. and 880° C., respectively, in order to releasehydrogen. Re-hydrogenation of these hydrides is, however, easy and theyabsorb hydrogen readily even under low hydrogen pressure.

[0005] From the group of unstable metal hydrides, AlH₃ and LiAlH₄ arethe most characteristic examples, having such a high equilibriumpressure of hydrogen that gaseous hydrogenation is in practiceimpossible. Only chemical reactions are effective for the formation ofthese unstable hydrides. On the other hand, both AlH₃ and LiAlH₄ desorbhydrogen readily at temperatures between 100° C. and 130° C. and withgood kinetics.

[0006] So far, the only materials which exhibit practical, reversibleproperties, i.e., hydrogenation/dehydrogenation at ambient conditions,for example, LaNi₅, FeTi or Ti—V, have a hydrogen capacity of less than2 wt. %, which is too low for practical applications.

DISCLOSURE OF THE INVENTION

[0007] The invention provides hydrogen storage compositions capable ofreversible operation at moderately elevated temperatures of 80-180° C.,typically 100-150° C., and having a hydrogen capacity exceeding 2 wt. %.

[0008] In accordance with one aspect of the invention there is provideda reversible hydrogen storage composition having a hydrogen capacityexceeding 2 wt. % and capable of reversible operation at temperatures of80-180° C. comprising i) an unstable AlH₃-based complex hydride alloyedby ball milling with: ii) at least one member selected from the groupconsisting of: a) an element that does not readily form a hydride in asolid-state form, b) a metal which forms a stable metal hydride, c) ahydride of a metal of b), and d) an unstable AlH₃ hydride complexdifferent from said complex i), said unstable AlH₃-based complexhydrides i) and ii)d) liberating hydrogen readily at temperatures of 100to 150° C.

[0009] In accordance with another aspect of the invention there isprovided a composition of the invention in a dehydrogenated state.

[0010] In accordance with yet another aspect of the invention there isprovided a method of supplying hydrogen comprising liberating hydrogenfrom a composition of the invention at a temperature of at least 80° C.with formation of a dehydrogenated form of the composition, removing theliberated hydrogen, and absorbing hydrogen in the dehydrogenated form toregenerate the AlH₃-based complex hydride as a future source ofhydrogen.

[0011] In this specification reference to a “dehydrogenated form” or“dehydrogenated state” refers to a form or state of the hydrogen storagecomposition of the invention resulting from liberation of hydrogen fromthe composition. It is not intended to indicate that completedehydrogenation has necessarily occurred, and contemplates both acomplete dehydrogenation and a partial dehydrogenation resulting fromliberation of at least part of the hydrogen content of the composition.

[0012] The reference to an AlH₃-based complex hydride refers to thosecomplex metal hydrides such as NaAlH₄ or LiAlH₄ which liberate hydrogenreadily at moderate temperatures of the order of 100 to 150° C., forminga dehydrogenated form or state from which the hydride form can not beregenerated with hydrogen gas, or which can only be regenerated underextreme conditions impractical in a reversible hydrogen storagecomposition. These AlH₃-based complex hydrides are sometimes referred toas being “unstable” in view of their ease in liberating hydrogen and thedifficulty in regeneration from the dehydrogenated form.

DETAILED DISCLOSURE OF THE INVENTION

[0013] The invention is based on the discovery, that properties of theseso-called unstable metal hydrides, which decompose easily but are verydifficult to re-hydrogenate, can be altered in such a way that therequired re-hydrogenation conditions are much more favourable and thehydride can be regenerated with hydrogen gas in a practical operation.

[0014] The alteration of the unstable metal hydride may be achieved bychanging the chemical composition of the hydride, accompanied bymechanical grinding.

[0015] More especially the invention is concerned with hydrides based onAlH₃-complex. AlH₃ is very unstable and decomposes spontaneously attemperatures above 100° C. Normally, AlH₃ can not be rehydrogenated,even at extremely high hydrogen pressures, after hydrogen has beenliberated from it. The same applies to other hydrides based on AlH₃complex, for example, LiAlH₄, NaAlH₄, Mg(AlH₄)₂ and Ca(AlH₄)₂. Thesehydrides offer very high hydrogen capacities, typically up to 7-wt. %,and desorb hydrogen easily at temperatures between 100° C. and 180° C.,but normally can not be rehydrogenated at hydrogen pressures lower than100 atmospheres.

[0016] In this invention, properties of such unstable hydrides arechanged by incorporating in them other elements or hydrides, typicallythe incorporation may be by alloying the components together by, forexample, mechanical grinding or ball milling.

[0017] A large variety of unstable AlH₃-based hydrides have beeninvestigated with different alloying components, and such alloying isfound to produce dramatic change in the hydrogenation properties of theAlH₃-based hydrides. As a consequence, the hydrides become reversiblefor practical applications because rehydrogenation can be performed atmuch lower hydrogen pressures.

[0018] Typical AlH₃-based complex hydrides employed in the invention maybe represented by formula (I):

M_(x)(AlH₃)_(y)H_(z)   (I)

[0019] wherein M is a metal; x is an integer of 1 to 3, y is an integerof 1 or 2, and z is equal to x or 2x. Preferred examples of M are Li,Na, Be, Mg and Ca, and preferably x is 1 or 3.

[0020] Suitable AlH₃-based complex hydrides for use in the inventioninclude LiAlH₄, NaAlH₄, Mg(AlH₄)₂, Be(AlH₄)₂, Zr(AlH₄)₂, Ca(AlH₄)₂,Li₃AlH₆ and Na₃AlH₆ all of which change their hydrogen sorptionproperties when mechanically ground or ball milled in the presence of atleast one member selected from the following Groups:

[0021] 1. elements that do not form hydrides in a solid-state form undernormal conditions, for example, metalloids such as B, C, Si, P and S,and metals such as Cr, Mn, Fe, Co, Ni, Cu, Mo, Zn, Ga, In and Sn;

[0022] 2. elements which form relatively stable metal hydrides, such asBe, Mg, Ca, Ti, V, Y, Zr and La;

[0023] 3. hydrides of the elements from Group 2 above such as BeH₂,MgH₂, CaH₂, TiH₂, VH₂, YH₂, ZrH₂ and LaH₂;

[0024] 4. other hydrides based on the AlH₃-complex.

[0025] These additions, alone or in mixtures, are able to change thesorption properties of the AlH₃-based complex hydrides. The mechanism ofthe change is not fully understood, but it is probable that differentmechanisms are involved with the different classes of additive.

[0026] The probable mechanisms of altering hydrogenation properties ofAlH₃based hydrides are as follows:

[0027] i) interstitial alloying of the AlH₃-based hydride.

[0028] This mechanism is most probable in the case of metalloids as, forexample, boron and carbon.

[0029] ii) substitutional alloying accompanied by catalysis.

[0030] This mechanism is expected to apply to most metal additions fromgroup 2 elements.

[0031] iii) synergetic effect of hydrogen sorption in mixtures ofhydrides.

[0032] This mechanism was found in mixtures of AlH₃-based hydrides withhydrides of groups 3 and 4 above and also as a result of ball millingwith elements from group 2 above. In the latter case, however, formationof the respective hydrides, listed in group 3, can occur duringhydrogenation/dehydrogenation cycling of the main AlH₃-based hydride.Depending on the addition, there are essentially two kinds of behaviourof hydride mixtures. One is of a kinetic character, when the basichydride does not react with the addition. In this case the addition actsas a hydrogen carrier or catalyst and improves the reaction kinetics.The second mechanism is based on formation of new complex hydrides ormore complicated hydride complexes. In this case thermodynamicproperties of the main AlH₃-based hydride are significantly altered,resulting in changed equilibrium pressures for hydride formation.

[0033] The above mechanisms for the improvement of hydrogen sorptionproperties of AlH₃-based complex hydrides, as a result of changes in thechemical compositions by means of mechanical alloying with additions,were studied in various AlH₃-based complex hydrides with a number ofadditions from the above groups of materials. Within one family ofadditions, the amounts of additions were varied.

[0034] Typically the molar ratio of the AlH₃-based complex hydride tothe addition was changed in the range between 10:1 to 1:3. Samples ofAlH₃based complex hydrides with no additions at all, but ball milled atthe same conditions as the samples with additions, were also studied. Asa general conclusion it was found that in each case ball milling aloneimproved kinetic properties of the AlH₃-based complex hydride, but ballmilling with additions improved them much more remarkably and evidentlycould change thermodynamical properties of the main hydride.

[0035] Although the detailed nature of these changes has not been fullydetermined, some general conclusions can be described as follows, inconnection with the mechanisms proposed above.

[0036] Interstitial character of alloying with metalloids is confirmedby x-ray diffraction analysis. For example, addition of C to NaAlH₄ inthe molar proportion of 1:1 does not change the x-ray diffractionpattern of NaAlH₄ and no other reflections were observed which couldindicate formation of other phases. Also, no reflections or halos fromcrystalline or amorphous carbon can be seen in the x-ray diffractionpattern. At the same time, however, this material exhibits hydrogenationproperties which differ dramatically from conventional NaAlH₄. Asreported previously [1,2] NaAlH₄ has such a high equilibrium pressure ofhydrogenation that it was normally impossible to rehydrogenate it afterdecomposition. Only recently Bogdanovic discovered a catalyst thatenabled rehydrogenation of NaAlH₄ [3, 4]. However, a very high hydrogenpressure of 150 atm was still necessary to perform absorption at 170° C.

[0037] A material according to the present invention, being a ballmilled mixture of NaAlH₄ and C exhibits reversible hydrogen sorptionproperties at much lower pressures and with much faster kinetics.Equilibrium pressure for this material is, for example, about two timeslower at 140° C., than the reported values for conventional NaAlH₄ [1,2]. This means that much lower hydrogen pressures are required toeffectively perform hydrogen absorption. Moreover, kinetics of thehydrogenation/-dehydrogenation cycles remarkably exceeds the reactionrates observed not only for the conventional NaAlH₄, but also for thecatalysed NaAlH₄ of the prior art [3, 4]. For example, the catalysedNaAlH₄ desorbs 2 wt. % of hydrogen at 160° C. within about 6 hrs. [3,4], while NaAlH₄ with C of the invention can desorb the same amount ofhydrogen within only 30 min. For comparison, conventional NaAlH₄ withoutcatalyst requires more than 50 hrs. at 160° C. to desorb 2 wt. % ofhydrogen.

[0038] In another example boron is found to be very effective inchanging the hydrogen sorption properties of different AlH₃-basedcomplex hydrides. In complex hydrides boron shifts the equilibriumpressure of hydrogen towards lower pressures, i.e., stabilizes theAlH₃-based complex hydride. This is very advantageous because as aresult these hydrides can effectively operate at lower hydrogenpressures.

[0039] Addition of silicon also results in significantly enhancedkinetics of hydrogenation cycling of the AlH₃-based complex hydrides.

[0040] Additions of metals, for example, Cu, Ni, Fe and Zn also improvesorption properties of AlH₃-hydride complexes. The presence of Cu, Fe orMn can be seen in the x-ray diffraction pattern, but with a clearlyreduced size of the metal grains, which is very advantageous from thepoint of view of the possible catalytic action of the additions.

[0041] AlH₃-based complex hydrides ball milled with elements whicheasily form hydrides, from group 2 above, are among the most interestingmaterials because of the variety of the possible combinations of thehydride mixtures. The additions can be introduced into the mixture inthe form of elements or their hydrides, according to groups 2 and 3above. Ball milling with these additions results in the formation ofhydride complexes with changed thermodynamical properties. For example,ball milling of NaAlH₄ with zirconium or with its hydride results insuch a change of the equilibrium pressure that effective absorption ofhydrogen can be performed at 60 to 80 atm instead of 150 atm., asreported previously for catalysed NaAlH₄ [3, 4]. Kinetics ofhydrogenation cycling are also many times faster at similar temperaturesthan catalysed NaAlH₄. Excellent hydrogen sorption properties appear tobe even more enhanced when the mixtures are in the nanocrystalline form,with the components being extremely finely intermixed.

[0042] Suitably the AlH₃-based complex hydride and the additive from oneor more of groups 1, 2, 3 and 4 above, have a particle size below 100μm, preferably below 50 μm.

[0043] The hydrogen storage compositions of the invention liberatehydrogen at a temperature of at least 80° C, generally 80 to 180° C. andtypically 100 to 180° C.

[0044] Hydrogen absorption into the dehydrogenated form of the hydrogenstorage composition of the invention is suitably carried out at atemperature of 80 to 150° C. typically 100 to 150° C., and a hydrogenpressure of 20 to 100, preferably 30 to 80 atm for a period of 0.25 to 5hours, preferably 0.5 to 3 hours.

[0045] The hydrogen storage compositions of the invention have ahydrogen capacity of 2 to 7, more usually 3 to 7 wt. %.

BRIEF DESCRIPTION OF DRAWINGS

[0046]FIG. 1 illustrates graphically desorption of hydrogen from ahydrogen storage composition of the invention, based on NaAlH₄ and C;

[0047]FIG. 2 illustrates graphically hydrogen absorption duringregeneration from the dehydrogenated form of the composition of FIG. 1;

[0048]FIG. 3 illustrates graphically hydrogen desorption from a hydrogenstorage composition of the invention, based on NaAlH₄ and Cu;

[0049]FIG. 4 illustrates graphically hydrogen absorption from a hydrogenstorage composition of the invention, based on NaAlH₄ and Zn;

[0050]FIG. 5 illustrates the diffraction pattern of a hydrogen storagecomposition of the invention derived from NaAlH₄ and Zr after long (18h) and short (1 h) milling time;

[0051]FIG. 6 illustrates graphically desorption of hydrogen from thestorage composition of FIG. 5; and

[0052]FIG. 7 illustrates graphically hydrogen desorption from a hydrogenstorage composition of the invention, based on NaAlH₄ and LiAlH₄.

EXAMPLES Example 1

[0053] NaAlH₄ was ball milled with an addition of C in a high-energyball mill SPEX 8000 (Trade-mark). The molar ratio of the hydride to Cwas 3:1. After ball milling the mixture was placed in an automated gastitration system and subjected to hydrogenation/dehydrogenation cycles.The material was able to desorb about 3 wt. % at temperatures as low as90°- 150° C. (FIG. 1), with an additional 1.5 wt. % being released atabout 160° C. Rehydrogenation was performed at about 80 atm at atemperature of 130° C. and was completed within 2 to 3 hours (FIG. 2).

Example 2

[0054] NaAlH₄ was ball milled in a high-energy ball mill, with additionsof metals that normally do not form metal hydrides, i.e. Cu and Zn. Inboth cases the molar ratio of the hydride to metal was 4:1. Thesubsequent hydrogenation/dehydrogenation cycles showed efficientdesorption and absorption at temperatures and under hydrogen pressure atwhich conventional NaAlH₄ could never operate (FIGS. 3 and 4).

Example 3

[0055] NaAlH₄ was ball milled with addition of zirconium, added at amolar ratio of 4:1. After ball milling the material exhibited extremelyfine microstructure which was seen in the x-ray diffraction pattern, allBragg's reflections being nearly vanished (FIG. 5). For comparison,x-ray diffraction pattern of another mixture of NaAlH₄+ Zr (3:1, shortermilling time) is also presented in FIG. 5. As compared to theconventional NaAlH₄, this material exhibits outstanding hydrogenationproperties with fast hydrogen desorption at temperatures between 110 and150° C. (FIG. 6) and rehydrogenation at hydrogen pressures of 50-80 atm.Similar hydrogenation performance is obtained when zirconium isintroduced in the form of zirconium hydride, ZrH₂, instead of Zr.

Example 4

[0056] LiAlH₄ ball milled with Mg was investigated by differentialscanning calorimetry (DSC, Perkin-Elmer). Comparison of the DSC tracesfor the hydrogen evolution showed that the sample containing magnesiumdesorbed hydrogen at higher temperatures than the sample withoutadditions. This means that stability of the hydride with magnesium wasincreased, leading to lower hydrogen pressures required forrehydrogenation. The same was confirmed by desorption measurements inthe gas titration system. Similar increase of the stability of LiAlH₄can be also obtained after ball milling with magnesium hydride insteadof magnesium.

Example 5

[0057] A mixture of two hydrides: NaAlH₄ and LiAlH₄ in a molar ratio of1:1, was ball milled in a high energy ball mill (SPEX 8000). As aresult, the mixture exhibited great improvement in the desorptionkinetics (FIG. 7), as compared to the conventional desorption of NaAlH₄and LiAlH₄.

References

[0058] [1] T. N. Dymova, N. G. Eliseeva, S. Bakum and Y. M. DergacheyDolk, Akad. Nauk SSSR, Vol. 215, p. 1369, 1974.

[0059] [2] T. N. Dymova, Y. M. Dergachev, V. A. Sokolov and N. A.Grechanaya Dolk, Akad. Nauk SSSR, Vol. 224, No. 3, p. 591, 1975.

[0060] [3] B. Bogdanovic and M. Schwickardi, J. Alloys and Comp., Vol.253, p. 1, 1997.

[0061] [4] B. Bogdanovic, German Pat. Appln. No. 195 26 434.7, 1995.

We claim:
 1. A reversible hydrogen storage composition having a hydrogencapacity exceeding 2 wt. % and capable of reversible operation attemperatures of 80-180° C. comprising i) an unstable AlH₃-based complexhydride alloyed by ball milling with: ii) at least one member selectedfrom the group consisting of: a) an element that does not readily form ahydride in a solid-state form, b) a metal which forms a stable metalhydride, c) a hydride of a metal of b), and d) an unstable AlH₃ hydridecomplex different from said complex i), said unstable AlH₃-based complexhydrides i) and ii)d) liberating hydrogen readily at temperatures of 100to 150° C.
 2. A composition according to claim 1 , wherein said memberis a said element a) being a metalloid selected from B, C, Si, P and S.3. A composition according to claim 1 , wherein said member is a saidelement a) being a metal selected from Cr, Mn, Fe, Co, Ni, Cu, Mo, Zn,Ga, In and Sn.
 4. A composition according to claim 1 , wherein saidmember is a said metal b) selected from Be, Mg, Ca, Ti, V, Y, Zr and La.5. A composition according to claim 1 , wherein said member is a saidhydride c) selected from BeH₂, MgH₂, CaH₂, TiH₂, VH₂, YH₂, ZrH₂ andLaH₂.
 6. A composition according to claim 1 , wherein said member is asaid hydride complex d).
 7. A composition according to claim 1 , whereinthe AlH₃-based complex hydride i) is of formula (I):M_(x)(AlH₃)_(y)H_(z)   (I) wherein M is a metal, x is an integer of 1 to3, y is an integer of 1 or 2, and z is equal to x or 2x.
 8. Acomposition according to claim 7 , wherein M is selected from Li, Na,Be, Mg and Ca and x is 1 or
 3. 9. A composition according to claim 1 ,wherein said unstable AlH₃-based complex hydride i) is selected fromLiAlH₄, NaAlH₄, Mg(AlH₄)₂, Be(AlH₄)₂, Zr(AlH₄)₂, Ca(AlH₄)₂, Li₃AlH₆ andNa₃AlH₆.
 10. A composition according to claim 1 , having a molar ratioof said hydride to said member of 10:1 to 1:3.
 11. A compositionaccording to claim 7 , having a molar ratio of said hydride to saidmember of 10:1 to 1:3.
 12. A composition according to claim 8 , having amolar ratio of said hydride to said member of 10:1 to 1:3.
 13. Acomposition according to claim 9 , having a molar ratio of said hydrideto said member of 10:1 to 1:3.
 14. A composition according to claim 1 ,wherein said member is alloyed with said hydride by ball milling to aparticle size below 100 μm.
 15. A composition according to claim 9 ,wherein said member is alloyed with said hydride by ball milling to aparticle size below 100 μm.
 16. A composition according to claim 1 ,which liberates hydrogen at a temperature above 80° C. to produce amaterial which is a dehydrogenated state of said composition, saidmaterial being adapted to absorb hydrogen to regenerate said hydrogenstorage composition.
 17. A composition of claim 16 , in saiddehydrogenated state.
 18. A method of supplying hydrogen comprisingliberating hydrogen from a reversible hydrogen storage compositionhaving a hydrogen capacity exceeding 2 wt. % and capable of reversibleoperation at temperatures of 80-180° C. comprising i) an unstableAlH₃-based complex hydride alloyed by ball milling with: ii) at leastone member selected from the group consisting of: a) an element thatdoes not readily form a hydride in a solid-state form, b) a metal whichforms a stable metal hydride, c) a hydride of a metal of b), and d) anunstable AlH₃ hydride complex different from said complex i), saidunstable AlH₃-based complex hydrides i) and ii)d) liberating hydrogenreadily at temperatures of 100 to 150° C., at a temperature of at least80° C. with formation of a dehydrogenated form of said composition,removing said liberated hydrogen, and absorbing hydrogen in saiddehydrogenated form to regenerate said composition as a future source ofhydrogen.
 19. A method according to claim 18 , wherein said temperatureis 80 to 180° C.
 20. A method according to claim 18 , wherein saidabsorbing of hydrogen is carried out at a temperature of 80 to 150° anda hydrogen pressure of 10 to 100 atmospheres.